Bulk superhard B-C-N nanocomposite compact and method for preparing thereof

ABSTRACT

Bulk, superhard, B-C-N nanocomposite compact and method for preparing thereof. The bulk, superhard, nanocomposite compact is a well-sintered compact and includes nanocrystalline grains of at least one high-pressure phase of B-C-N surrounded by amorphous diamond-like carbon grain boundaries. The bulk compact has a Vicker&#39;s hardness of about 41-68 GPa. It is prepared by ball milling a mixture of graphite and hexagonal boron nitride, encapsulating the ball-milled mixture, and sintering the encapsulated ball-milled mixture at a pressure of about 5-25 GPa and at a temperature of about 1000-2500 K.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally superhard materials and moreparticularly to a superhard compact of nanocrystalline grains of atleast one high-pressure phase of B-C-N embedded in a diamond-likeamorphous carbon matrix and to a method for preparing the superhardcompact.

BACKGROUND OF THE INVENTION

Superhard materials have a Vickers hardness (Hv), i.e. an indentationhardness, of at least 40 GPa and are widely used as abrasives fordrilling, cutting, and other machining applications. Superhard materialsoften include boron, carbon, nitrogen and oxygen because these lightelements have a small atomic radius and form strong and directionalcovalent bonds that produce tight, three-dimensional networks withextreme resistance to external shear.

Diamond is the hardest superhard material currently known, with an Hv ofabout 70-100 GPa. However, the actual performance of diamond as anabrasive is somewhat limited. Diamond is an unsuitable abrasive formachining ferrous alloys and has limited applications for high-speedcutting because it is converted into graphite in the presence of oxygenat temperatures over 800° C.

Cubic BN (cBN) is another important superhard material. While cBN iswidely used for machining fully hardened steels and exhibits much betterthermal stability than diamond, it is only about half as hard(H_(v)=45˜50 GPa) as diamond.

Superhard materials for industrial use are often in the form of sinteredpolycrystalline composites that incorporate microcrystalline grains ofdiamond or cubic boron nitride. The grains of this composite are tens tohundreds of micrometers in size, and usually include vacancies,dislocations, and other imperfections that multiply and propagate toform microcracks within individual crystals of a grain, and also alonggrain boundaries. As the microcracks grow, the materials deform andfracture.

Recently, a new class of materials known as superhard nanocomposites hasbeen reported. Superhard nanocomposites contain superhardnanocrystalline grains embedded in an amorphous matrix. The amorphousmatrix provides amorphous grain boundaries that absorb vacancies anddislocations, reduces the surface energy and residual stress among thegrains, and permits the relaxation of mismatches between adjacent grainsof different phases. While a number of superhard nanocomposites havebeen reported, no superhard nanocomposite bulk compact having theVickers hardness of diamond has yet been prepared. Thus, there remains aneed for a superhard nanocomposite compact with improved hardness,strength, and performance.

Therefore, an object of the present invention is to provide a bulksuperhard nanocomposite compact with a high Vickers hardness.

Another object of the invention is to provide a method for preparing abulk superhard nanocomposite compact with a high Vickers hardness.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION.

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention includes a superhardnanocomposite compact. The compact consists essentially ofnanocrystalline grains of at least one high-pressure phase of B-C-Nsurrounded by amorphous diamond-like carbon grain boundaries.

The invention also includes a process for preparing a bulk superhardnanocomposite compact consisting essentially of nanocrystalline grainsof at least one high-pressure phase of B-C-N surrounded by amorphous,diamond-like grain boundaries. To prepare the compact, a mixture ofgraphite and hexagonal boron nitride is ball-milled. The ball-milledmixture contains amorphous and/or nanocrystalline graphitic carbon andboron nitride. The ball-milled mixture is encapsulated and sintered at apressure of about 5-25 GPa and at a temperature of about 1000-2500 K.

The invention is also a bulk, superhard nanocomposite compact preparedby the process of ball-milling a mixture of graphite and hexagonal boronnitride until the mixture is transformed into amorphous and/ornanocrystalline graphitic carbon and boron nitride. The ball milledmixture is encapsulated and sintered at a pressure of about 5-25 GPa andat a temperature of about 1000-2500 K.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiment(s) of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 shows a high magnification SEM image of the precursor materialused to prepare the bulk, superhard nanocomposite compact of theinvention;

FIG. 2 shows an x-ray diffraction pattern of the precursor material ofFIG. 1;

FIG. 3 shows and a Raman spectrum of the precursor material of FIG. 1;

FIG. 4 shows diffraction patterns plotted as intensity versus 2-ThetaAngle;

FIG. 5 shows synchrotron x-ray patterns in the energy dispersive modefor three compacts of the invention;

FIG. 6 shows a synchrotron XRD pattern plotted as intensity versusd-spacing for a compact of the invention prepared at 20 GPa and 1900 C;

FIG. 7 shows a high-resolution transmission electron microscopy (HRTEM)image of a compact of the invention; and

FIG. 8 shows an electron energy-loss spectroscopy (EELS) spectrum foramorphous, ball-milled starting material and an EELS spectrum for acompact of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes a superhard B-C-N nanocomposite compactand a method for preparing the compact. The compact includesnanocrystalline grains of at least one high-pressure B-C-N phaseembedded in a diamond-like amorphous matrix. The practice of theinvention can be further understood with the accompanying figures.Similar or identical structure is identified using identical callouts.

The compact is produced by first preparing a ball-milled mixture ofgraphite and hexagonal boron nitride (hBN). A tungsten carbide vial andtungsten carbide milling balls were used for the ball milling procedure.FIG. 1 shows a high magnification scanning electron microscope (SEM)image of the ball-milled powdered mixture after 34 hours of ballmilling. As the SEM image shows, the mixture is dark, has a grain sizeless than 0.1 micron, and does not appear to have a crystallinemorphology.

FIG. 2 includes two x-ray diffraction spectra. The upper spectrum is ofthe mixture of graphite and hexagonal boron nitride before ball milling.The lower spectrum is of the ball-milled mixture after 34 hours of ballmilling. As FIG. 2 shows, the ball-milled mixture appears to beamorphous.

FIG. 3 shows three Raman spectra. The upper spectrum is of hexagonalboron nitride (hBN), the middle spectrum is of graphite, and the lowerspectrum is of the ball-milled mixture of graphite and hBN. The lowerspectrum includes a peak at 1350 cm⁻¹ (a defect/disorder peak) and apeak at 1580 cm⁻¹ that is assigned to a graphitic phase. Importantly,the lower spectrum suggests that along with a graphite phase, theball-milled mixture also includes grains of nanocrystalline particlesand the intensity ratio between the 1350 cm⁻¹ peak and the 1580 cm⁻¹peak indicates that the grain size is about 2-3 nanometers (nm).

A sample of the ball-milled powder having the lower Raman spectrum ofFIG. 3 was encapsulated in a cylindrical platinum capsule and compressedusing a multi-anvil to a pressure of about 5-25 GPa. At this elevatedpressure, the encapsulated powder was sintered at a temperature of about1000-2500 K for 2-120 minutes. After the sintering period, the capsulewas brought to room temperature and decompressed to ambient pressure.The compact was removed is from the capsule and the ends of the compactwere polished with a diamond abrasive. The resulting polished compactwas a well-sintered cylindrical bulk compact having a height of about1.5-mm and a diameter of about 1.2-mm. These dimensions are a reflectionof the dimensions of the capsule used. Obviously, compacts of differentsizes and shapes depend on the size and shape of the capsule and thecell assembly used. A larger capsule and cell assembly would require alarger sample size and result in a larger compact. Likewise, smallercapsules and cell assemblies could also be used to prepare smallercompacts.

Several examples of the bulk compact of the invention were preparedaccording to the conditions summarized in Table 1 below.

TABLE 1 Precursor powder Pres- Sintering Sintering Vickers com- suretemperature Time hardness Entry position (GPa) (K) (minutes) (GPa) Color1 BCN 20 1300 2 Black 2 BCN 20 2100-2400 10  Light yellow 3 BC₂N 6-81500 120  Black 4 BC₂N 10 2000 5 Black 5 BC₂N 15 1800 5 50 6 BC₂N 152000 5 41 Black 7 BC₂N 16 2100 5 50 Gray- white 8 BC₂N 20 2200 5 62Light yellow 9 BC₂N 25 2130 10  Light yellow 10  BC₂N 25 2300 60  Lightyellow 11  BC₄N 20 2300 5 68 brown

As Table 1 shows, the compacts varied in color. Some were translucent,while others were opaque. Some were black (entries 1, 3, 4, and 6) whileothers were gray-white (entry 7), brown (entry 11), and light yellow(entries 2, 8, 9, and 10. The color seems to be dependent on therelative amount of graphite, the pressure, and the sinteringtemperature).

The Vickers hardness for several of them (entries 5, 6, 7, 8, and 11)were measured and determined to be in the range of about 41-68 GPa. TheHv for any particular compact of the invention appears to be dependentupon the precise composition of the precursor powder and on thesynthesis conditions. Three precursor powder compositions were used.Entries 1 and 2 (BCN) employed a composition of a 1:1 molar ratio ofgraphite:hBN (i.e. 1 part graphite and 1 part hBN). Entries 2-9 (BC₂N)employed a composition 2:1 molar ratio of graphite:hBN (i.e. 2 partsgraphite and 1 part hBN). Entry 11 (BC₄N) employed a powder compositionof a 4:1 molar ratio of graphite:hBN (i.e. 4 parts graphite and 1 parthBN. Pressures varied from about 6 GPa to about 25 GPa, sinteringtemperatures varied from about 1300 K (entry 1) to about 2400 K (entry2), and sintering times varied from 2 minutes (entry 1) to about 120minutes (entry 3). The preparation of several of these compacts is nowdescribed.

EXAMPLE 1

The compact of entry 2 was synthesized as follows. About 5 grams of aball milled mixture of a 1:1 molar ratio of graphite:hBN were prepared.About 3 mm³ of the ball-milled mixture was placed into a platinumcapsule. Using a split-sphere multi-anvil press, the encapsulatedmixture was subjected to a pressure of about 20 GPa and then sintered ata temperature of about 2100-2400 K for about 10 minutes. The resultingcompact was light yellow in color.

EXAMPLE 2

The compact of entry 5 was synthesized as follows. About 3 mm³ of theball-milled mixture described in Example 1 was placed into a platinumcapsule. Using the anvil press of example 1, the encapsulated mixturewas subjected to a pressure of about 15 GPa, and then sintered at atemperature of about 2100 K for about 5 minutes. The resulting bulkcompact had a measured Vickers hardness was 50 GPa.

EXAMPLE 3

The compact of entry 8 was synthesized as follows. About 3 mm³ of theball-milled mixture described in Example 1 was placed into a platinumcapsule. Using the anvil press of Example 1, the encapsulated mixturewas subjected to a pressure of about 20 GPa, and then sintered at atemperature of about 2200 K for about 5 minutes. The resulting bulkcompact of the invention was light yellowish in color, translucent, andhad a measured Vickers hardness of 62 GPa.

EXAMPLE 4

The compact of entry 9 was synthesized as follows. About 3 mm³ of theball-milled mixture described in Example 1 was placed into a platinumcapsule. Using the anvil press of Example 1, the encapsulated mixturewas subjected to a pressure of about 25 GPa, and sintered at atemperature of about 2130 K for about 10 minutes. The resulting bulkcompact of the invention was light yellow in color.

EXAMPLE 5

The compact of entry 10 was synthesized as follows. About 3 mm³ of theball-milled mixture described in Example 1 was placed into a platinumcapsule. Using the anvil press of Example 1, the encapsulated mixturewas subjected to a pressure of about 25 GPa, and sintered at atemperature of about 2300 K for about 60 minutes. The resulting bulkcompact of the invention was light yellow in color.

EXAMPLE 6

Compact number 11 was synthesized as follows. A mixture of a 4:1 molarratio of graphite:hBN was prepared. About 3 mm³ of the ball-milledmixture was placed into a platinum capsule. Using the anvil press ofExample 1, the encapsulated mixture was subjected to a pressure of about20 GPa and sintered at a temperature of about 2200 K for about 5minutes. The resulting bulk compact of the invention was brownish andtranslucent, with a measured Vickers hardness was 68 GPa.

The microstructure and composition of the compact of the invention wasprobed using a variety of techniques. While optical microscopy andscanning microscopy were relatively uninformative, the granularstructure of the compact of the invention was revealed using theAdvanced Photon Source (APS) at Argonne National Laboratory, whichprovided monochromatic synchrotron x-ray diffraction in angle dispersivemode. The compact was interrogated using a narrow (5×7 μm²), collimatedX-ray beam (λ=0.4146 Å). The x-rays by the compact were collected usingan image plate in angle-dispersive mode to cover a 2-Theta (2Θ) anglerange up to 32 degrees, which corresponds to a minimum d-spacing of 0.77Å. Changing the position of the beam spot on the compact had no effecton the diffraction pattern, which indicated that the sample washomogeneous in structure and composition.

FIG. 4 shows X-ray diffraction patterns that are plotted as intensityversus 2-Theta Angle for the BC₂N bulk compact of the inventionsynthesized at a pressure 20 GPa and a sintering temperature of 2200 K.The upper diffraction pattern was obtained when the compact was rockedwith an amplitude of 5 μm. The middle diffraction pattern is for thenon-rocking, stationary compact, and the lower pattern is a standarddiffraction pattern of cerium oxide (CeO₂), which is included in orderto indicate the resolution of the x-ray diffraction instrument. As FIG.4 shows, the peaks of the middle pattern are about 5-6 times as broad asthe peaks of the lower pattern. As the upper pattern shows, the peaksbroadened even more (8-10 times as broad as the lower pattern) when thesample was rocked. From these observations, it was concluded that thecompact includes nanocrystalline grains. Using Scherrer's equation, thegrain size was estimated at about 4-8 nm.

The major diffraction peaks shown in FIG. 4 for the compact of theinventionare consistent with a face-centered-cubic (fcc) zinc-blende(ZnS) structure with a unit cell parameter a=3.595(7) Å. This unit celldimension lies between diamond (a=3.567 Å) and cBN (a=3.616 Å), and isin close agreement with the unit cell parameter reported by E. Knittleet al. in “High Pressure Synthesis, Characterization, and Equation ofState of Cubic C-BN Solid Solutions,” Phys. Rev. B. vol. 51, 1995, pp.12149-12156; by T. Komatsu et al. in “Creation of Superhard B-C-NHeterodiamond Using Shock Wave Compression Technology,” J. Mater.Processing Technology, vol. 85, 1999, pp. 69-73; and by W. Utsumi etal., in “In situ X-Ray and TEM Observations on the Phase Transitions ofBC₂N Under Static Pressures,” Proceedings of AIRAPT-18, Beijing, 2001,p. 186. From these papers, it appears that E. Knittle et al., T. Komatsuet al. and W. Utsumi et al. were unable to produce the well-sintered,bulk superhard nanocomposite compact of the present invention.

FIG. 5 shows synchrotron x-ray diffraction patterns in the energydispersive mode for three compacts of the present invention. The compactthat produced the top x-ray diffraction pattern was sintered at 15 GPaand 2000 K. The compact that produced the middle x-ray diffractionpattern was sintered at 16 GPa and 2100 K. The compact that produced thebottom x-ray diffraction pattern was sintered at 20 GPa and 2200 K. Eachpattern includes the <111>pc and <220>pc peaks of the fcc lattice.Compacts of the invention prepared at 20 GPa were about 15-20% harderthan those prepared at 15-16 GPa. Compacts that were prepared at stilllower pressures produced x-ray diffraction patterns that exhibitedadditional peaks and apparent peak splitting that was most noticeablefor the <200>pc peak. This may suggest the existence of a superlattice,lower symmetry, or an additional phase. Generally, compacts prepared athigher pressures appear to have higher symmetry.

FIG. 6 shows the synchrotron XRD pattern, plotted as intensity inarbitrary units versus d-spacing in angstroms, of a compact prepared at20 GPa and 2200 K. The top left inset shows that the <111>peak can befitted by a sum of two curves; a broadened crystalline peak (curve A)and an amorphous hump (curve B). The plus (+) indicates the dataobserved and the dark solid line is the curve calculated from thefitting of curve A with curve B.

FIG. 7 shows high-resolution transmission electron microscopy (HRTEM)image of a BC₂N compact of the present invention synthesized at apressure of 20 GPa and a sintered at a temperature of 2200 K for 5minutes. The HRTEM image confirms the presence of 3-8 nm nanocrystallinegrains with an average size of about 5 nm, which is consistent with thesynchrotron X-ray diffraction pattern. The zinc-blende fcc structure,apparent from FIG. 4, is confirmed by the electron diffraction patternshown in the top right inset of FIG. 7. FIG. 7 includes an enlargedimage of a grain. The enlarged image appears to include an apex of aregular icosohedron, a regular polyhedron with 20 triangular faces andfive-fold symmetry. Lines have been added to more clearly show thesefeatures. Also according to FIG. 7, the grain boundaries between thenanocrystalline grains appear to be amorphous.

The chemical composition and chemical bonding of individual grains ofthe compact were determined using electron energy-loss spectroscopy(EELS), a powerful technique for obtaining local chemical compositionand chemical bonding information in materials composed of lightelements. Samples of the compact were prepared for EELS by anion-thinning process or by directly impacting the sample into finepowder. The results were the same for both sample preparation methods. Anarrow (3-4 nm) focused electron beam was used to probe the chemicalcomposition and bonding of individual nanocrystalline grains. FIG. 8shows an upper EELS spectrum for the amorphous, ball-milled startingmaterial and a lower EELS spectrum for a single nanocrystalline grain.The lower EELS spectrum includes the K-edges for B, C, and N, whichconfirms that the grain is composed of a single ternary B-C-N phaserather than a mixture of a diamond phase and a cBN phase. The upper EELSspectrum for the amorphous ball-milled starting material includes π*peaks at the Kedges for the B, C, and N. The appearance of these π*peaks in the amorphous starting material suggests the presence ofsp²-hybridized hexagonal ring fragments. The π* peaks of the EELSspectrum of the amorphous material do not appear in the EELS spectrum ofthe compact.

The chemical composition of the grain boundaries was examined using acombination of HRTEM and EELS. Unexpectedly, the grain boundaries arecomposed of amorphous, diamond-like carbon (DLC). DLC is typicallyproduced by such methods as vacuum arc or pulsed laser deposition, andhas stimulated great interest because of its high hardness, chemicalinertness, thermal stability, wide optical gap, and negative electronaffinity. It is believed that the bulk, superhard, nanocomposite compactof the invention is the first bulk, nanostructured compact reported withDLC grain boundaries, which are believed to contribute significantly tothe mechanical strength of the compact.

The enhanced fracture toughness of the compact of the invention islikely due, at least in part, to the substantial absence of vacanciesand dislocations in the individual nanocrystalline grains, and also tothe difficulty of microcrack propagation through the amorphous grainboundaries separating the grains.

The effects of using a ball-milled amorphous material as the precursormaterial were examined by preparing compacts from a different precursormaterial: a mixture of graphite and hexagonal boron nitride (hBN) thathad not been subjected to ball milling. Compacts prepared without ballmilling the mixture of graphite and hBN did not include nanocrystallinegrains of BC₂N. Instead, these compacts included segregated phases ofdiamond and cBN. The presence of segregated phases was first suggestedby optical microscopy, more strongly indicated by x-ray diffractionspectra that showed twin-peaks of all the major x-ray diffraction peaks,and finally confirmed by Raman spectra that showed the characteristicpeaks of diamond and cBN.

The invention also includes machining tools of the bulk superhardcompact of the invention. The compact could be used for drilling,cutting, puncturing, and other types of machining.

In summary, the invention includes a well-sintered, bulk, superhard,nanocomposite compact and a method for preparing the bulk compact. Thebulk compact includes nanocrystalline grains of at least onehigh-pressure phase of B-C-N embedded in a diamond-like amorphous carbonmatrix. A variety of analytical techniques show that the bulk compactcontains nanocrystalline grains of B-C-N having a diamond-likestructure. The structure symmetry and Vickers hardness (Hv=50-73 GPa) ofthe bulk compact of the invention appear to increase with the pressureused to prepare the compact. The Vickers hardness of several examples ofthe bulk compact was higher than that for cBN (47 GPa, see T. Taniguchiet al. in “Sintering of cubic boron nitride without additives at 7.7 GPaand above 2000° C., J. Mater. Res., vol. 14, pp. 162-169, 1999) and forhBN single crystals (45-50 GPa, see Handbook of Ceramic Hard Materials,R. Riedel ed., pp. 104-139, Wiley-VCH Verlag GmbH, D-69469, Weinheim,2000) and were very close to the hardness of diamond (70-100 GPa) It isexpected that the compact of the invention is more stable at hightemperatures than diamond and that machining tools employing the compactof the invention will not react with ferrous metals during high-speedcutting.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. Commercially available autofocus laser end effectors,for example, could be used instead of the laser end effectors describedherein.

The embodiment(s) were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

What is claimed is:
 1. A bulk, superhard, nanocomposite compactconsisting essentially of nanocrystalline grains of at least onehigh-pressure phase of B-C-N surrounded by amorphous diamond-like carbongrain boundaries.
 2. The compact of claim 1, wherein said compact has aVickers hardness of about 41-68 GPa.
 3. The compact of claim 1, whereinsaid compact has a Vickers hardness of about 50-68 GPa.
 4. The compactof claim 1, wherein said compact has a VicKers hardness of about 62-68GPa.
 5. The compact of claim 1, wherein said compact has a Vickershardness of 68 GPa.
 6. A bulk, superhard, nanocomposite compactconsisting essentially of nanocrystalline grains of at one high-pressurephase of B-C-N surrounded by amorphous diamond-like carbon grainboundaries produced by the process comprising the steps of: (a) ballmilling a mixture of graphite and hexagonal boron nitride to produce amixture of amorphous and/or nanocrystalline graphitic carbon and boronnitride; (b) encapsulating the ball-milled mixture; and (c) sinteringthe encapsulated ball-milled mixture at a pressure of about 5-25 GPa anda temperature of about 1000-2500 K, thereby producing a bulk, superhardnanocomposite compact consisting essentially of nanocrystalline grainsof B-C-N surrounded by amorphous diamond-like carbon grain boundaries.7. The compact of claim 6, wherein the ball milled mixture of graphitehexagonal boron nitride consists essentially of about 1-4 parts graphiteto about 1 part hexagonal boron nitride. 8.The compact of claim 7,wherein the ball milled mixture of graphite and hexagonal boron nitrideconsists essentially of about 2 parts graphite to about 1 part hexagonalboron nitride.
 9. The compact of claim 7, wherein the ball milledmixture of graphite and hexagonal boron nitride consists essentially ofabout 4 parts graphite to about 1 part hexagonal boron nitride.
 10. Thecompact of claim 7, wherein the encapsulated ball-milled mixture issintered at a pressure of about 10-25 GPa and at a temperature of about2000-2500 K.
 11. The compact of claim 7, wherein the encapsulatedball-milled mixture is sintered at a pressure of about 15-25 GPa and ata temperature of about 2000-2500 K.
 12. The compact of claim 7, whereinthe encapsulated ball-milled mixture is sintered at a pressure of about16-25 GPa and at a temperature of about 2100-2500 K.
 13. The compact ofclaim 7, wherein the encapsulated ball-milled mixture is sintered at apressure of about 20-25 GPa and at a temperature of about 2000-2500 GPa.14. The compact of claim 7, wherein the encapsulated ball-milled mixtureis sintered at a pressure of about 20-25 GPa and at a temperature ofabout 2100-2400 K.
 15. The compact of claim 7, wherein the encapsulatedball-milled mixture is sintered at a pressure of about 20 GPa and at atemperature of about 2000-2400 K.
 16. The compact of claim 7, whereinthe encapsulated ball-milled mixture is sintered at a pressure of about25 GPa and at a temperature of about 2100-2300 K.
 17. The compact ofclaim 7, wherein said compact has a VicKers hardness of about 41-68 GPa.18. The compact of claim 7, wherein said compact has a Vickers harnessof about 50-68 GPa.
 19. The compact of claim 7, wherein said compact hasa Vickers hardness of about 62-68 GPa.
 20. The compact of claim 7,wherein said compact has a Vickers hardness of 68 GPa.
 21. The bulk,superhard, nanocomposite compact of claim 6, wherein step (b) comprisesencapsulating the ball-milled mixture in capsule comprising platinum,gold, rhenium, or boron nitride.